Unexpected burst of star formation re-ignites the Phoenix Cluster

Composite image of the Phoenix Cluster, consisting of ultraviolet (UV), optical, and X-ray images. The inset combines optical and UV light, zooming in on the central galaxy of the cluster, where rapid star-formation is in progress.

Galaxy clusters can contain hundreds of galaxies, and the ones at their centers are the largest galaxies known to astronomers. These huge objects are the product of successive mergers between smaller galaxies, a process that has exhausted the raw materials for the formation of stars. That is, in every case but one—a new set of observations have revealed one of the largest and hottest galaxy clusters yet seen, and its central galaxy has an unexpectedly high rate of star formation. This finding may provide new insights into the history of galaxy clusters and the formation of structure in the Universe.

Galaxy clusters are the largest objects in the Universe bound by their own gravity; rich clusters contain hundreds of galaxies. While most of a cluster's mass is in the form of dark matter, each also has a significant amount of hot plasma filling the space between galaxies. This intracluster medium (ICM) is very bright in X-ray light, due to internal temperatures greater than 10 million Kelvins (107 K).

In most cases, the ICM is diffuse, but there are some hot cores where the density is high enough that the plasma can cool by emitting radiation. Once cooled, the matter will fall to the center of the cluster due to gravity, creating what's called a cooling flow.

According to theoretical models, the incoming mass of cooling flows are usually offset by strong jets from the supermassive black holes (SMBHs) in the large galaxies lying near the centers of many clusters. The balance between cooling flows and SMBH feedback drives the evolution of galaxy clusters.

Researchers used observations (both new and archival) from 10 different observatories to characterize the galaxy cluster SPT-CLJ2344-4243, known colloquially as the "Phoenix Cluster" for its location in the Phoenix constellation. The "phoenix" designation is also apt because of the extraordinary high rate of star formation in a class of galaxy where such activity usually had ceased long before.

The researchers combined data from observatories including the South Pole Telescope (SPT), Chandra X-ray telescope, the Blanco Telescope at Cerro Tololo (incidentally where dark energy was first discovered), the Magellan telescope, and the Herschel Space Observatory. The reason for this was to obtain data across the electromagnetic spectrum, from radio to X-ray. This allowed the astronomers to characterize the cluster's shape, the star formation rate, and the behavior of the SMBH at the heart of the Phoenix.

The detailed look at this cluster has provided the strongest observation of a cooling flow to date. What they found was striking: the Phoenix Cluster emitted 8.2×1038 watts in X-rays alone, half as luminous as the next-brightest known galaxy cluster. Most of that radiation came from the ICM, and it resulted in a cooling flow as the plasma in the ICM gave up energy and fell inward.

Based on infrared and ultraviolet data, the researchers estimated that, as it reached the central galaxy, the infalling matter drove a star formation rate equivalent to 740 new Suns per year (though most of new stars will be less massive than the Sun).

Additionally, they studied the behavior of the SMBH in the central galaxy of the Phoenix Cluster. The evidence pointed to a very active black hole—expected from the largest galaxies in other clusters—but one obscured by dust, making it appear more like the black holes in dust-filled star-producing galaxies. In other words, the central galaxy in the Phoenix Cluster exhibited characteristics of both normal central galaxies and star-forming galaxies found in other environments. The dust helps shield the environment from the black hole, which keeps it from forming jets that balance out the incoming matter.

Finally, the shape of the cluster was found to be remarkably spheroidal, meaning it likely hadn't undergone a merger in the recent past. Mergers can also set off a wave of star formation, so ruling this out provides another indication that the cooling flow was driving star formation.

The whole picture from all the different data sources appeared consistent: the rapid star formation in the central galaxy of the Phoenix Cluster was the result of a cooling flow of gas, uninhibited by strong feedback from the SMBH. Since the cluster's shape appeared regular, the star formation was unlikely to have been caused by two clusters merging. Together, this data suggests a new way galaxy clusters can give birth to new stars—a significant addition to our understanding of the history and evolution of the Universe.

(Thanks to Peter Edmonds of the Chandra X-ray Center at the Harvard-Smithsonian Center for Astrophysics for providing additional information and the image.)

12 Reader Comments

Whenever I read articles on the happenings of the universe I always assume that phrases like "high rate of star formation" refer to something like one star every 100 years. That this article clarifies it to mean up to 740 individual new Suns per year was mind blowing. ...although I guess all that activity is still spread out over an almost incomprehensible distance.

This intracluster medium (ICM) is very bright in X-ray light, due to internal temperatures greater than 10 million Kelvins (10^7 K).

ok wait a damn minute. Are you saying the SPACE IN BETWEEN some galaxies is not only not empty, it has enough material to reach a temperature of 10 million K?! Am I reading that right?!

I thought the only space with any considerable temperature was on or near physical bodies. That's fascinating that I couldn't set a certain course for Alderaan because some areas of space would frikkin VAPORIZE me.

aardarf wrote:ok wait a damn minute. Are you saying the SPACE IN BETWEEN some galaxies is not only not empty, it has enough material to reach a temperature of 10 million K?! Am I reading that right?!

One of the things I remember from astrophysics, a long time ago in a galaxy far far away (lol!) was that temperature and pressure were 2 sides of the same coin. A high temperature gas is also a high pressure gas. You're unlikely to survive the pressure even before you get to the 10 million K part of the gas in-flow. Of course that assumes you survive the Xray bombardment.

Sounds like this galaxy cluster is the astronomical definition of Hell. Makes the orbit of Mercury seem frosty!

This intracluster medium (ICM) is very bright in X-ray light, due to internal temperatures greater than 10 million Kelvins (10^7 K).

ok wait a damn minute. Are you saying the SPACE IN BETWEEN some galaxies is not only not empty, it has enough material to reach a temperature of 10 million K?! Am I reading that right?!

I thought the only space with any considerable temperature was on or near physical bodies. That's fascinating that I couldn't set a certain course for Alderaan because some areas of space would frikkin VAPORIZE me.

The gas is that hot but the gas is also thin. As long as your ship doesn't hit too many gas molecules and absorb too much heat it will be fine. The x-ray on the other side is another story.

Describing a cosmic process as "being re-ignited" gives one the impression of "now-ness". But there is no now-ness in the cosmos. "Now" is a human-centric concept involving human-scale time frames. Things in space take decades, centuries and millennia to "occur". Stars have lives of up to billions of years - they don't go from not-a-star-yet to now-it's-a-star in seconds, minutes, hours or days. Or weeks, or months, either. The headline is more misleading than concise.

And that's not to mention that at cosmic scales, due to the speed of light limitation, you can't escape the idea that distance is time and nothing we can see "out there" is really happening "now".

No, but we are receiving the information now. As far as I'm aware Astronomy data follows Human timescales, because its being interpreted by Humans. As far as we are concerened it is happening now, because now is when we are collecting the data in the form of various radiation emmisions.

Also the headline "Unexpected burst of star formation, which happened 5.7 billion years ago, re-ignited the Phoenix Cluster" isn't quite as snappy.

Agree.It's all relative. Sure this all happened a long time ago, but we still are recording live as this happens or as the information reaches us. Besides as long as we don't have direct connections to those places it doesn't matter whether we adjust the time or not. Recording the progress and the time of progress is more important than any other time discrepancy.

Describing a cosmic process as "being re-ignited" gives one the impression of "now-ness". But there is no now-ness in the cosmos. "Now" is a human-centric concept involving human-scale time frames. Things in space take decades, centuries and millennia to "occur". Stars have lives of up to billions of years - they don't go from not-a-star-yet to now-it's-a-star in seconds, minutes, hours or days. Or weeks, or months, either. The headline is more misleading than concise.

And that's not to mention that at cosmic scales, due to the speed of light limitation, you can't escape the idea that distance is time and nothing we can see "out there" is really happening "now".

I don't think I agree with your reading there. Who said it happened in minutes? You're the one projecting a human time scale onto the cosmos.

You shouldn't think of a match when you read "re-ignites". You should instead think of an unimaginably ancient galaxy cluster full of old stars that should be running down their (astronomically huge) clocks where, instead, there are something like 740 new stars appearing every year.